1. Field of the Invention
The present invention relates to an analog optical transmission system adapted to broadband communication, more specifically, FTTH (Fiber To The Home) service, that enables multiplication of digital data signals used in data communication service such as internet and analog signals used for broadcasting service.
2. Related Background Art
In recent years, users for the broadband communication that multiplexes the data communication service such as internet and electric mail, video broadcasting service, and so on continue to increase rapidly.
In particular, the number of users of the FTTH (Fiber To The Home) service increases remarkably, the FTTH service drawing from an end station that is the last repeating station of an existing communication network to a subscriber's home via optical fibers in a variety of broadband communication systems. The FTTH service is not only an excellent system from a point of view of communication speed, communication quality, and so on, but also excellent for a service extension such as wavelength multiplexing; thus, it is expected as a main communication system to be in charge of enlarged growth of the broadband communication. That is, the current FTTH service is only used for transmission/reception of digital data signals, while the optical fiber has the great feature that enables simultaneous transmission of wavelength multiplexing. Accordingly, it is expected to further develop to wavelength multiplying services founded on an optical fiber network currently under maintenance. In this way, it is pointed out that various services such as distributions of not only the digital signals used in the internet and the like, but also the image signals based on an analog transmission system may be achieved without great plant and equipment investments.
So far, when an analog transmission such as the CATV system is employed, there are many cases that light from a DFB laser is modulated externally, in order to enhance the transmission length, to achieve an excellent linearity and avoid the distortion by chirps by DFB laser. However, an use of such an external modulator may cause an increase of the number of components and/or an increase at manufacturing cost. On the other hand, when the DFB laser is directly modulated, a system can be achieved with low cost, but easily causes a distortion due to an FM response in the laser, and further is subject to effects of the chromatic dispersion to be occurred in the transmission line; consequently, a transmittable distance is restricted (currently, at most several kilometers).
In the aforementioned analog optical transmission system, it is necessary to reduce the effect of the chromatic dispersion by means of a dispersion compensating device arranged at a predetermined position of the transmission line. For example, patent references 1–3 disclose constitutions enabling dispersion compensation on analog transmission. Additionally, the patent reference 2 discloses an technique utilizing a dispersion compensating fiber as a dispersion compensating device to multiple branched transmission lines, which is not suitable for analog transmission.
For example, the Japanese Patent Application Laid-Open Nos. JP-A-2003-152646 (patent reference 1), JP-A-07-135493(patent reference 2), JP-A-2001-144354 (patent reference 3), JP-A-04-260007 (patent reference 4) disclose various dispersion-compensating technique in conventional analog optical transmission system, respectively.